This invention relates to apparatus for controlling the electron beam in a beam index color television receiver and, more particularly, to such apparatus wherein inherent time delays therein are compensated such that as the beam moves to a particular color element on the display screen of the cathode ray tube, the beam is modulated with a color control signal that is associated with that color element.
Beam index color television receivers are known wherein the display screen of the cathode tube in that receiver is provided, in addition to the usual beam-excitable color elements, such as red (R), green (G) and blue (B) phosphor stripes, with periodic index stripes. The phosphor stripes, as is conventional, are arrayed in RGB triads, repetitively across the display screen so as to be scanned by the electron beam as the latter effects a horizontal line scan in, for example, left-to-right traverse. As the electron beam scans the color phosphor stripes, it also scans the index stripes which, typically, also are phosphor stripes so as to emit light when excited by the scanning electron beam. In order to prevent light from the scanned index stripes from interfering with the displayed television picture, the index stripes are disposed on one surface of a thin metal layer and the color phosphor stripes are disposed on the opposite surface of this thin metal layer, which layer is substantially transparent to the scanning electron beam but blocks the light which is emitted by the phosphor index stripes. A photo-detector responds to each excited phosphor index stripe to produce a periodic signal whose frequency is equal to the frequency at which the phosphor index stripes are excited. Thus, as the electron beam scans a horizontal line across the display screen, the photo-detector generates a periodic index signal.
Examples of beam index color television receivers are disclosed in copending Applications Ser. Nos. 969,861, filed Dec. 15, 1978; 969,975, filed Dec. 15, 1978; and 972,236, filed Dec. 22, 1978, all assigned to the assignee of the instant invention.
The index signal which is derived from the scanning of the aforementioned phosphor index stripes is used to gate red, green and blue color control signals onto, for example, the first grid of the cathode ray tube in successive time sequence. Since the index signal is derived from the scanning of the electron beam, the index signal is related to the scanning velocity of that beam. Thus, the gating of the respective color control signals, referred to as color switching, desirably is synchronized with the beam velocity. This means that when the beam moves into scanning alignment with, for example, a red phosphor element, the red control signal is gated so as to modulate the beam with red signal information. Then, as the beam moves into proper scanning alignment with the green phosphor element, the red control signal is interrupted and the green control signal is gated so as to modulate the beam. Similarly, when the beam next moves into proper scanning alignment with a blue phosphor element, the green control signal is interrupted and the blue control signal is gated to modulate the beam. The foregoing gating sequence is repeated so that, as the beam scans the red, green and blue phosphor elements, it is concurrently and synchronizingly modulated with the red, green and blue color information.
In a beam index color television receiver of the type described in the above-mentioned copending applications, red, green and blue gates are provided for the red, green and blue color information signals, respectively, and each of these red, green and blue gates is opened individually and in sequence as the beam scans a horizontal line. A phase-locked loop is provided to synchronize the gating signals, that is, the signals which are used to open the red, green and blue gates sequentially, with the index signal. Thus, if the index signal undergoes a change in frequency due to, for example, a change in the scanning velocity of the beam, the red, green and blue gates nevertheless will be opened at the proper time, that is, at the times that the beam moves into proper scanning alignment with the red, green and blue phosphor elements, respectively. However, the phase-locked loop suffers from an inherent time delay. That is, when the output of the phase-locked loop is "locked" to the index signal, there actually may be a small time difference between these signals. Also, the output of the phase-locked loop cannot follow instantaneously all changes in the index signal because of the inherent time delay. Furthermore, the circuitry that is used to produce the red, green and blue gating signals, as well as the red, green and blue gates themselves, exhibit an inherent time delay. The overall time delay of the color switching control circuitry is known as a static delay, and can effect errors in the color switching sychronization. That is, and with reference to, for example, the red gate, this gate should open to gate the red control, or color information, signal when the beam moves into proper scanning alignment with a red phosphor element. Unfortunately, because of the aforementioned inherent static delay of the color switching control circuitry, the red gate might open at a slightly delayed time. This means that the beam already may be advanced with respect to the proper scanning alignment thereof relative to the red phosphor element at the time that the red gate opens. This red gate might remain open for a brief period of time when the beam then moves into scanning alignment with an adjacent green phosphor element. Thus, erroneous color synchronization may occur.